Modularizing Shiny app code

Description

As Shiny applications grow larger and more complicated, app authors frequently ask us for techniques, patterns, and recommendations for managing the growing complexity of Shiny application code. A Shiny module is a piece of a Shiny app. It can’t be directly run, as a Shiny app can. Instead, it is included as part of a larger app (or as part of a larger Shiny module–they are composable).